DL saves on fireworks.
Also, WDW upcharges guests thru their many 'luxe' hotels wherein most of the upcharge is due to proximity to MK (even tho the rooms ain't that luxe.)
MK is just a piece of the larger WDW pie. The other three theme parks and all the other entertainment synergizes with MK.
Who'd go to DAK if DAK was just a single theme park out in the world by itself with no other 'linked' theme parks nearby to hit up in one trip? (Yes, some would, but, it would definitely not be as popular).
DAK is profitable because it's part of the WDW network. And MK, with less rides than DL profits from anchoring that network.
When you look at just *rides*, the four WDW parks have as many as DL/DCA. So, to hit all the rides, you are enticed to stay longer and spend more. Especially in a Disney hotel. While WDW tickets are cheaper the more days you by, the hotels are the same rate each day (mostly). Then on top of having the same number of rides (albeit spread out more), WDW has a lot more non-ride attractions.
Comparing DL to just MK is a false comparison. Each has their own network of supporting cast. And WDW has a lot more supporting cast, which make MK as profitable over all as part of that network.
People forget that when they focus on MK 'expanding.' Every time a new ride/attraction is added or refreshed at DAK, EPCOT, or DHS... that's an expansion to MK, too.